I hope no one is using Stoneskin or Clemency to keep themselves alive, you can't block or parry while casting.

Having a lot of experience in progression in this game, I can say this:

PEOPLE WHO CARE ABOUT PARRY(AND SHOULD): Progression raiders, solo tanks, tanks with very aggressive Cleric Stance healer, and tanks with poor healers.

PEOPLE WHO DON'T CARE ABOUT PARRY: Not the above.